Scientists have long believed that Mercury has water ice in its polar craters, which are shadowed at all times

NASA's MESSENGER spacecraft has found further evidence supporting that water ice exists on Mercury.

The MESSENGER spacecraft, which entered Mercury's orbit last year, used its Mercury Dual Imaging System to take pictures throughout 2011 and 2012. NASA scientists have observed these photographs, and confirmed that certain radar-bright features at the north and south poles on Mercury lie within shadowed regions. This falls in line with the water ice hypothesis.

Scientists have long believed that Mercury has water ice in its polar craters, which are shadowed at all times. This belief stems from the fact that Mercury's tilt to almost zero on its rotational axis, meaning that there are certain areas that never see sunlight.

There are other bits of evidence that point to the existence of water ice on Mercury, such as the excess hydrogen measured at Mercury's north pole by MESSENGER's Neutron Spectrometer, the detailed models of Mercury's topography by the Mercury Laser Altimeter (MLA), and measurements of the reflectance of Mercury's polar deposits at near-infrared wavelengths.

Now, the detailed images produced by MESSENGER's MLA, which uses laser pulses to image the topography on Mercury, has found that ice is buried beneath an abnormally dark material across a majority of the deposits where the surface is a bit too warm. Also, dark patches with a diminished reflectance is covered by a layer of insulation.

Scientists believe the dark material is a combination of complex organic compounds that came from Mercury's collision with comets and asteroids, and it as darkened by the radiation on Mercury's surface.

In addition, they found that the north polar regions have irregular dark and bright deposits at near-infrared wavelength near the north pole.

"These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice," said Gregory Neumann of the NASA Goddard Flight Center. "Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice."